1. Field of the Invention
The present invention relates to light-emitting elements of a current excitation type. The present invention also relates to light-emitting devices and electronic devices each of which has the light-emitting device. More specifically, the invention relates to light-emitting elements of a current excitation type having a long lifetime. Further, the invention relates to light-emitting devices and electronic devices each of which uses the light-emitting device.
2. Description of the Related Art
In recent years, research and development have been extensively conducted on light-emitting elements using electroluminescence. As a basic structure of these light-emitting elements, a structure where a substance having light-emitting property is interposed between a pair of electrodes is used. By application of a voltage to this element, light emission from a substance having light-emitting property can be obtained.
Since such a light-emitting element is a self-luminous type, there are advantages such as higher visibility of a pixel than visibility of a liquid crystal display, and unnecessity of a backlight. Accordingly, such a light-emitting element is considered to be suitable as a flat panel display element. In addition, such a light-emitting element can be manufactured to be thin and light, which is a great advantage. Moreover, the light-emitting element has a feature that response speed is extremely fast.
Furthermore, since such a light-emitting element can be formed into a film form, planar light emission can be easily obtained by formation of a large-area element. This characteristic is difficult to be obtained by a point light source typified by an incandescent lamp or an LED, or a line light source typified by a fluorescent lamp. Therefore, the light-emitting element has a high utility value as a plane light source that can be applied to lighting or the like.
The light-emitting elements using electroluminescence are classified roughly in accordance with whether they use an organic compound or an inorganic compound as a substance having light-emitting property. In the present invention, a substance having light-emitting property is an organic compound.
In a case where the substance having light-emitting property is an organic compound, by application of voltage to the light-emitting element, electrons and holes are injected from the pair of electrodes into the layer including an organic compound having light-emitting property to cause current flow. Then, by recombination of these carriers (electrons and holes), the organic compound having light-emitting property gets in an excited state, and light is emitted when the excited state returns to a ground state.
Because of such a mechanism, this kind of light-emitting element is referred to as a light-emitting element of a current excitation type. It is to be noted that an excited state formed by an organic compound can be a singlet excited state or a triplet excited state. Light emission from the singlet excited state is referred to as fluorescence, and light emission from the triplet excited state is referred to as phosphorescence.
In order to overcome many problems derived from materials of such a light-emitting element and to improve its element characteristics, improvement in an element structure, material development, and so on are carried out.
For example, in Non-Patent Document 1, a hole blocking layer is provided so that a light-emitting element using a phosphorescent material efficiently emits light.
[Non-Patent Document]
    Tetsuo TSUTSUI et al., Japanese Journal of Applied Physics, vol. 38, L1502-L1504 (1999)